halcyon days

From halcyon, from Latin Alcyone, from Ancient Greek Ἀλκυόνη (Alkuónē), daughter of Aeolus and wife of Ceyx. When her husband died in a shipwreck, Alcyone threw herself into the sea whereupon the gods transformed them both into halcyon birds (kingfishers). When Alcyone made her nest on the beach, waves threatened to destroy it. Aeolus restrained his winds and kept them calm during seven days in each year, so she could lay her eggs. These became known as the "halcyon days," when storms do not occur. Today, the term is used to denote a past period that is being remembered for being happy or successful.

来源：wiktionary